Snap cursor - wrong positioning with three vertices
It's stated pretty clearly in the Blender manual, but if someone omits this source it'll be here:


  The Cursor to Selected option is also affected by the current Pivot Point. For example:
  
  
  With the Bounding Box Center pivot point active, the Cursor to Selected option will snap the 3D cursor to the center of the bounding
  box surrounding the objects’ origins.
  When the Median Point pivot point is selected, Cursor to Selected will snap the 3D cursor to the median of the object origins.